1. Field of the Invention
This invention relates generally to the wire bonding technology used for making electronic components. More specifically, the present invention relates to a method of bonding an aluminun wire for electrically connecting a between semiconductor chip and a lead or between a pair of leads.
2. Description of the Prior Art
As is well known, leadframes are commonly used for manufacturing various electronic components such as transistors, ICs, and etc. The leadframe, which is an integral piece made of a thin metal sheet, includes a plurality of chip mounting portions each for mounting a semiconductor chip, and a plurality of leads associated with the respective chip portions. After mounting the chip, a thin metal wire is bonded to the chip and to a corresponding lead of the leadframe, and a resinous package is molded to enclose the chip together with the wire and part of the lead.
When the metal wire is made of gold or copper, the bonding of the wire is performed by the so-called thermo compression bonding method wherein an end of the wire is pressed against the workpiece (namely, the chip or the lead) while heating to a relatively high temperature. If necessary, ultrasonic vibration may be additionally applied to the wire end to increase the bonding strength.
When the metal wire is made of aluminum, on the other hand, it is known that the positive application of heat causes weakening of the wire. Further, it is also known that the aluminum wire is easily oxidized by the positive application of heat. In view of this, the bonding of the aluminum wire has been conventionally performed by the so-called ultrasonic bonding method wherein an end of the aluminum wire is pressed against the workpiece under the application of ultrasonic vibration but without positive application of heat. In such a bonding method, heat is generated only frictionally at the position of contact between the workpiece and the ultrasonically vibrated wire end, and the thus generated heat causes bonding of the wire relative to the workpiece.
It has been found that the ultrasonic wire bonding method provides a weaker bonding strength as the surface roughness of the workpiece increases. In other words, if the surface roughness of the workpiece is high, the wire end, which has been ultrasonically bonded, may easily come off the workpiece.
Particularly, the leadframe as the workpiece is known to have an innumerable number of sharply edged slip lines (Luders' lines) which are formed at the time of rolling a metal sheet material, thereby increasing the surface roughness of the leadframe. If the wire end is ultrasonically vibrated in a direction transversing the slip lines, the wire end is subjected t material removal by the abrasive action of the slip lines. Therefore, care must be taken to render the direction of the ultrasonic vibration parallel to the extending direction of the slip lines, consequently reducing the designing freedom.
In view of these problems, Japanese Patent Application Laid-open No. 58-35950 discloses a leadframe which is plated with nickel to have a surface roughness of no more than 0.5 micrometers, thereby preventing a bonding strength decrease which would result from an increase of the surface roughness provided by the presence of the slip lines. However, the plating treatment is excessively time-taking and costly if performed to the degree of achieving a surface of roughness of no more than 0.5 micrometers.